Semiconductor wafers typically include a controlled concentration of a dopant to produce desired electrical characteristics. Such doped wafers are typically produced by adding a calculated amount of dopant to a polycrystalline semiconductor and melting the dopant and semiconductor together in a crucible in a Czochralski-type crystal-growing furnace. The polycrystalline silicon and the dopant mix together in a liquid state to produce a molten mixture having a desired dopant concentration that is proportional to the concentration desired in the finished wafer. A single-crystal ingot is pulled from the molten mix.
Some dopants, such as antimony, have a vapor pressure sufficiently high to cause the concentration of dopant in the melt to change significantly as the semiconductor and dopant are melting together in the furnace. The change in dopant concentration cannot be accurately predicted, so the ability to accurately produce crystals having desired electrical properties is limited. To overcome this problem, several techniques have been developed for adding high vapor pressure dopants to the semiconductor after it is melted. Such techniques are difficult to perform, however, because the molten semiconductor (the "melt") must be maintained in an inert atmosphere in the crystal-growing furnace.
A Czochralski crystal-growing furnace typically consists of two separately sealable vacuum-tight sections: a pull chamber, which has space for enclosing the ingot as it is grown and includes a seed cable or shaft for lowering and raising a seed crystal, and a furnace tank, which contains a crucible containing the melt.
A common method of adding a pelletized dopant in a B--B form to the melt entails using a special apparatus or delivery device suspended from the seed cable. The special apparatus is lowered to a fixed point above the melt and the pellets are released to the melt. After the special apparatus is removed from the seed cable and a seed crystal is attached, the seed crystal is lowered to the melt, and crystal pulling can begin.
This method has several disadvantages. Proper dopant drop is difficult to achieve consistently. Problems such as splashing and evaporation are not uncommon. Furthermore, this method requires the pull chamber be opened and evacuated twice. Such extra steps slow the crystal-growing operation and allow the dopant concentration to change through evaporation. Such steps are also labor intensive and increase the risk of a vacuum leak, which can contaminate the melt. The dopant delivery device can also bring contamination into the furnace. Another disadvantage is that the dopant delivery device is expensive and subject to wear or breakage.
Japanese patent application J 62-153188, assigned to Mitsubishi Metal KK, describes doping a semiconductor melt by attaching a dopant to a seed crystal and then dipping the seed and dopant into the melt. Dipping the seed into the melt, without appropriate pre heating causes thermal stress within the seed which can result in cracks or structural defects. Pre-heating of the seed may cause premature dopant drop causing a melt splash.
The Mitsubishi patent describes two methods for attaching the dopant to the seed. One method requires boring a transverse hole into the seed crystal near its unchucked end, melting the dopant, and solidifying the dopant around the seed and in the hole. Machining the seed, such as by boring a hole, requires additional process steps. The second attachment method uses a ring of dopant that is placed over the seed and kept from dropping off by a block of silicon laser-welded to the bottom of the seed. The laser-welded block of silicon separates from the seed after the block, seed, and dopant are dipped into the melt. This attachment or supporting method is complex and requires additional process steps and equipments.
Japanese patent application J 60-171291, assigned to Furukawa Electric Company, describes a dopant in the form of a thin electrical current-carrying wire attached to a seed crystal. The seed contacts the molten semiconductor, causing the dopant wire to melt and thereby interrupt the current flow. The interrupted current flow automatically causes a computer to begin the crystal-pulling procedure. The disadvantages of contacting the seed to the molten semiconductor are described above. This method also requires forming the dopant into a wire and attaching electrodes to the dopant within the high temperature crystal-growing environment.